I. Field of the Invention
This invention relates to the catalytic reforming of naphthas and gasolines for the improvement of octane.
II. The Prior Art
Catalytic reforming, or hydroforming, is a well established industrial process employed by the petroleum industry for improving the octane quality of naphthas or straight run gasolines. In reforming, a multi-functional catalyst is employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, notably alumina. Noble metal catalysts, notably of the platinum type, are currently employed, reforming being defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst.
Platinum has been widely commercially used in recent years in the production of reforming catalysts, and platinum-on-alumina catalysts have been commercially employed in refineries for the last few decades. In the last decade, additional metallic components have been added to platinum as promoters to further improve the activity or selectivity, or both, of the basic platinum catalyst, e.g., iridium, rhenium, both iridium and rhenium, tin, and the like. Some catalysts possess superior activity, or selectivity, or both, as contrasted with other catalysts. Platinum-rhenium catalysts by way of example possess admirable selectivity as contrasted with platinum catalysts, selectivity being defined as the ability of the catalyst to produce high yields of C.sub.5.sup.+ liquid products with concurrent low production of normally gaseous hydrocarbons, i.e., methane and other gaseous hydrocarbons, and coke.
In a reforming operation, one or a series of reactors, or a series of reaction zones, are employed. Typically, a series of reactors are employed, e.g., three or four reactors, these constituting the heart of the reforming unit. Each reforming reactor is generally provided with a fixed bed, or beds, of the catalyst which receive downflow feed, and each is provided with a preheater or interstage heater, because the reactions which take place are endothermic. A naphtha feed, with hydrogen, or recycle hydrogen gas, is co-currently passed through a preheat furnace and reactor, and then in sequence through subsequent interstage heaters and reactors of the series. The product from the last reactor is separated into a liquid fraction, and a vaporous effluent. The former is recovered as a C.sub.5.sup.+ liquid product. The latter is a gas rich in hydrogen, and usually contains small amounts of normally gaseous hydrocarbons, from which hydrogen is separated and recycled to the process to minimize coke production.
The sum-total of the reforming reactions, supra, occurs as a continuum between the first and last reactor of the series, i.e., as the feed enters and passes over the first fixed catalyst bed of the first reactor and exits from the last fixed catalyst bed of the last reactor of the series. The reactions which predominate between the several reactors differ dependent principally upon the nature of the feed, and the temperature employed within the individual reactors. In the initial reaction zone, or first reactor, which is maintained at a relatively low temperature, conditions are established such that the primary reaction involves the dehydrogenation of cyclohexanes to produce aromatics. The isomerization of naphthenes, notably C.sub.5 and C.sub.6 naphthenes, also occurs to a considerable extent. Most of the other reforming reactions also occur, but only to a lesser, or smaller extent. There is relatively little hydrocracking, and very little olefin or paraffin dehydrocyclization occurs in the first reactor, or reaction zone. Within the intermediate reactor(s), or zone(s), the temperature is maintained somewhat higher than in the first, or lead reactor of the series, and the primary reactions in the intermediate reactor, or reactors, involve the isomerization of naphthenes and paraffins, dehydrogenation of naphthenes to yield aromatics, and dehydrocyclization of C.sub.8.sup.+ paraffins to yield aromatics. Where, e.g., there are two reactors disposed between the first and last reactor of the series, some dehydrogenation of naphthenes may, and usually does occur, at least within the first of the intermediate reactors, or first portion of the reaction zone. There is usually some hydrocracking, at least more than in the lead reactor of the series, and there is more olefin and paraffin dehydrocyclization. The third reactor of the series, or second intermediate reactor, is generally operated at a somewhat higher temperature than the second reactor of the series. The naphthene and paraffin isomerization reactions generally continue in this reactor, and there is a further increase in paraffin dehydrocyclization, and more hydrocracking. In the final reactor, or final reaction zone, which is operated at the highest temperature of the series, paraffin dehydrocyclization, particularly the dehydrocyclization of the short chain, notably C.sub.6 and C.sub.7 paraffins, is the primary reaction. The isomerization reactions continue, and there is more hydrocracking in this reactor than in any of the other reactors of the series.
The activity of the catalyst gradually declines due to the build-up of coke. Coke formation is believed to result from the deposition of coke precursors such as anthracene, coronene, ovalene, and other condensed ring aromatic molecules on the catalyst, these polymerizing to form coke. During operation, the temperature of the process, or of the individual reactors, is gradually raised to compensate for the activity loss caused by the coke deposition. Eventually, however, economics dictate the necessity of reactivating the catalyst. Consequently, in all processes of this type the catalyst must necessarily be periodically regenerated by burning off the coke at controlled conditions.
Two major types of reforming are generally practiced in the multi-reactor units, both of which necessitate periodic reactivation of the catalyst, the initial sequence of which requires regeneration, i.e., burning the coke from the catalyst. Reactivation of the catalyst is then completed in a sequence of steps wherein the agglomerated metal hydrogenation-dehydrogenation components are atomically redispersed. In the semi-regenerative process, a process of the first type, the entire unit is operated by gradually and progressively increasing the temperature to maintain the activity of the catalyst caused by the coke deposition, until finally the entire unit is shut down for regeneration, and reactivation, of the catalyst. In the second, or cyclic type of process, the reactors are individually isolated, or in effect swung out of line by various manifolding arrangements, motor operated valving and the like. The off-oil catalyst is regenerated to remove the coke deposits, and then reactivated while the other reactors of the series, which contain the on-oil catalyst, remain on stream. A "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, until it is put back in series. Because of the flexibility offered by this type of "on-stream" catalyst regeneration, and reactivation, cyclic operations are operated at higher severities than semiregenerative operations, viz., at higher temperature and lower pressures.
Various improvements have been made in such processes to improve the performance of reforming catalysts in order to reduce capital investment or improve C.sub.5.sup.+ liquid yields while improving the octane quality of naphthas and straight run gasolines. New catalysts have been developed, old catalysts have been modified, and process conditions have been altered in attempts to optimize the catalytic contribution of each charge of catalyst relative to a selected performance objective. Nonetheless, while any good commercial reforming catalyst must possess good activity, activity maintenance and selectivity to some degree, no catalyst can possess even one, much less all of these properties to the ultimate degree. Thus, one catalyst may possess relatively high activity, and relatively low selectivity and vice versa. Another may possess good selectivity, but its selectivity may be relatively low as regards another catalyst. Platinum-rhenium catalysts, among the handful of successful commercially known catalysts, maintain a rank of eminence as regards their selectivity; and they have good activity. Platinum-iridium catalysts have also been used commercially, and these on the other hand, are extremely active, and have acceptable selectivity. However, iridium metal is very expensive, and in extremely short supply. Therefore, despite the advantages offered by platinum-iridium catalysts the high cost, and lack of availability raise questions regarding the commercial use of iridium-containing catalysts. The demand for yet better catalysts, or ways to use presently known catalysts nonetheless continues because of the existing world-wide shortage in the supply of high octane naphtha, and the likelihood that this shortage will not soon be in balance with demand. Consequently, a relatively small increase in the C.sub.5.sup.+ liquid yield, or decreased capital costs brought about by the use of catalysts with lesser loadings of precious metals, e.g., decreased iridium loadings, can represent large credits in commercial reforming operations.
Catalysts have been staged in various ways in catalytic reforming processes to achieve one performance objective, or another. Some perspective regarding such processes is given, e.g., in U.S. Pat. No. 4,436,612 which was issued on Mar. 13, 1984, to Oyekan and Swan, reference being made to Columns 3 and 4, respectively, of this patent. Both platinum-iridium and platinum-rhenium catalysts have been staged in one manner or another to improve reforming operations. Regarding the staging of platinum-rhenium catalysts, reference is made to U.S. Pat. No. 4,440,626-8 which issued on April 3, 1984, to U.S. Pat. No. 4,425,222 which issued on Jan. 10, 1984, and to U.S. Pat. No. 4,427,533 which issued Jan. 24, 1984. These patents, as well as U.S. Pat. No. 4,436,612, relate generally to processes wherein platinum-rhenium catalysts are staged, the amount of rhenium relative to the platinum being increased in the downstream reactors, i.e., in the final or tail reactor of the series, and in the intermediate reactor(s) of the series.